1. Field
The technology described herein relates to image detection apparatus and methods, such as metal oxide semiconductor (MOS) imagers and pixels.
2. Related Art
Conventional MOS imagers are known, and have a structure like that shown in FIG. 1. The imager 100 includes a focal plane array 101 formed by multiple pixels 102 arranged in rows and columns. The focal plane array 101 includes N rows and M columns of pixels 102. Each pixel 102 includes a photodetector which detects incident radiation in the visible spectrum and produces an output signal representative of the amount of incident radiation. The output signals of the pixels are provided to an image signal processor 104, which processes them to produce an image. The output signals of pixels 102 are provided serially to the image signal processor 104 under control of row multiplexer circuitry 106 and column multiplexer circuitry 108.
In a MOS imager, the pixels 102 are implemented using MOS technology. FIG. 2 illustrates a circuit schematic of one such pixel. The pixel 102 includes a photodetector 202, and additional circuitry to condition and control readout of the photodetector output to column line 204 as the output of pixel 102. The photodetector 202 is a photodiode, and is operated in a reverse-biased mode, having one terminal grounded and the other terminal connected to a positive rail Vdd by a precharge transistor 206. The precharge transistor 206 is an n-channel MOS field effect transistor (MOSFET), having its body grounded. The drain of precharge transistor 206 is connected to the positive rail Vdd and the source is connected to the photodetector 202.
As mentioned, the output of the pixel 102 is provided to the column line 204. Pixel 102 is one of multiple pixels in the focal plane array 101, with each pixel 102 in a column sharing a single column line 204 for providing their output signals. The output signal of photodetector 202 is not provided directly to column line 204, but rather it is buffered by a source follower 208, which is an n-channel MOSFET having its body grounded, and its gate connected to the photodetector 202.
The pixels of the focal plane array are read out by row, with each pixel providing its output to its respective column line. A row select signal is applied to all the pixels of the row when the output signals of the pixels of that row are to be read out. A row select transistor 210 in the pixel 102 is turned on by the row select signal, connecting source follower 208 to column line 204. A load (not shown) on the column line 204 causes the source follower 208 to operate as a source follower, with the voltage at its source roughly tracking the voltage at its gate.